1. Technical Field
The invention relates to electrochromic devices as used, for example, in so-called variable transmission windows or variable reflection mirrors, and in particular to counter-electrode materials for such devices.
2. Background Art
Electrochromic devices are known to have successive layers of electrochromic, electrolyte and counter-electrode materials. The device may have first and second laminar substrates each covered on one side with an electrically conducting film, the layers interposed between the two substrates with the film covered sides innermost. Alternatively, the device may have one laminar substrate covered on one side with an electrically conducting film, the layers being carried on this film covered side with a further electrically conducting film applied over the exposed layer. The most common substrate material is glass, but plastics materials, like acrylic, may also be used.
By way of example, the electrically conductive films may be indium doped tin oxide, the electrochromic material may be tungsten trioxide, the counter-electrode material may be cerium titanium oxide and the electrolyte material may be a suitable polymer to which lithium perchlorate has been added.
A tungsten trioxide/cerium titanium oxide device can be changed between bleached and colored states by altering the applied electrical potential, that is, the potential applied via the electrically conductive films (acting as electrodes) across the electrochromic, electrolyte and counter-electrode layers. The polarity of the potential dictates the direction of transfer of cations (provided by the lithium perchlorate) through the electrolyte material, between the electrochromic and the counter-electrode materials. The cation transfer is reversible. When reduced, or in other words when cations are inserted, the electrochromic material is colored, whereas, when oxidized (when cations are de-inserted), it is virtually colorless. Conversely, the counter-electrode material is chosen because it is virtually colorless when either reduced or oxidized, or at least any coloring on reduction is indiscernible.
A tungsten trioxide/cerium titanium oxide device can be varied from a blue colored state to a pale yellow "colorless state".
Other electrochromic/counter-electrode material combinations may work in reverse, with the electrochromic layer coloring on oxidation, and different combinations can produce different colors and degrees of color change. There are also devices wherein a single layer acts as both the counter-electrode and the electrically conducting film. Furthermore, there are devices, such as those available from the Gentex company, which have a single material which functions as the electrochromic, counter-electrode and electrolyte layers.
The changeability of an electrochromic device lends itself to use in, amongst other applications, a window where variable transmission characteristics are required. These are seen as being of particular use in integrated energy management systems for buildings; one idea being to modulate the solar gain of the building to maximize energy benefits. For instance, by coloring the window during the hottest part of a summer's day, the amount of solar radiation entering a building can be minimized, and on dull winter days the window can be bleached so as to make best use of the available natural light.
The degree of coloration of an electrochromic device is related to the quantity of cations inserted into the electrochromic material and hence, in the case of an electrochromic material coloring on the insertion of cations, the extent of reduction, which is dictated by the amount of charge passed; the greater the charge passed, the deeper the color. One of the limiting factors on the amount of charge passed is the charge storage capacity of the counter-electrode material. For instance, in a device which has a tungsten trioxide electrochromic layer and a cerium titanium oxide counter-electrode layer, the depth of the blue coloration attainable is restricted by the tendency of the cerium titanium oxide to saturate at a charge density well below that which the tungsten trioxide can tolerate. Thus, the tungsten trioxide effectively has unutilized charge storage capacity.
In addition, the dynamic range of the device, that is the ratio of the colored to bleached state optical transmission, is preferably as wide as possible. Most effective use of a management system controlled electrochromic window is achieved if the device has as wide a dynamic range as possible. Optimization of the dynamic range is assisted by having the counter-electrode layer as near as possible equally optically transmitting in both the colored and bleached states of the device.
A counter-electrode material also needs to have good long term cycling stability and good electrochemical kinetics.
WO 89/12844 (EIC Laboratories Inc) discloses a counter-electrode material composed of a mixture of metal oxides in combination with an electrochromic material coloring on reduction. Proposed in WO 89/12844 are mixed oxides of materials such as vanadium or chromium either together or with oxides of niobium, tantalum or titanium. However, WO 89/12844 is directed to counter-electrode materials which complement the electrochromic material, that is to say, counter-electrode materials that are colored when oxidized and colorless when reduced, and the specific examples deal only with niobium/vanadium or chromium/vanadium oxides and their characteristics.